(1) Field of the Invention
The present invention relates to matrix microparticles for the controlled release of agricultural actives, and more particularly to methods of producing lignin-based matrix microparticles for the controlled release of agricultural actives.
(2) Description of the Related Art
Pesticides, herbicides, plant growth regulating compounds and other related compounds are widely used to protect plants from diseases and pests and ultimately to increase crop yield or value. In addition to the potential benefits that such compounds promise, however, many of these materials are toxic to humans and other animals. Some can be harmful to the plants they are intended to protect. Thus, the consequences of unanticipated contact with such compounds over a long term or at high concentrations is undesirable. Moreover, because such compounds are complex molecules, most of which must be chemically synthesized, they are often expensive to produce, and can be chemically fragile. Therefore, in addition to the potential environmental harm that can be caused by leaching, blowing and other movement of the materials away from the area of initial application, some of these compounds are quickly degraded by ultraviolet (UV) light. These losses reduce the effectiveness of the compound and increase the amount that must be applied in order to provide a desired benefit.
One strategy for managing the safety and effectiveness of many of these biologically active compounds has been to provide them as controlled release formulations. Such formulations provide the active within a structure which limits the rate of transfer of the active into the surrounding environment and minimizes the movement of the active away from the site of application. General information on controlled release formulations for agricultural actives can be found in: Controlled-Release Delivery Systems for Pesticides, H. B. Scher, Ed., Marcel Dekker, Inc., NY (1999), Microencapsulation, Benita, S., (Ed.), Marcel Dekker, Inc., New York (1996), Controlled Delivery of Crop-Protection Agents, Wilkins, R. M., (Ed.), Taylor & Francis Ltd., London (1990), and Fernandez-Perez, M. et al., J. Agric. Food Chem., 46:3828 (1998), among others.
Common forms of controlled release formulations include microcapsules, microparticles and granules. Generally, microcapsules are considered to be particles of 1-100 microns in size that are composed of a distinct wall and a core that contains the active. Microparticles is a term that is generally used to describe matrix particles of 1-100 microns in size that have the active more or less uniformly distributed or dispersed within the matrix. Granules are matrix particles that are 0.2-2 mm in size with the active more or less uniformly distributed or dispersed throughout the matrix.
Each of these controlled release forms has advantages and disadvantages. For example, microcapsules that are formed by coating small solid particles of an active with a barrier material, often a polymer, are often of uneven shape and have uneven coating thickness over the surface of the particle—some even having exposed surfaces of the active. Accordingly, it is often difficult to assure predictable and even release of the active from such coated particles. Some coated particles permit high levels of the active at the surface and this can increase the exposure of handlers to the active and can result in rapid loss of the active upon application. These same disadvantages are also present in granules that have been produced by absorption of the active onto a carrier material.
Microcapsules having regular spherical shape and uniform walls can be formed by in situ polymerization of a polymeric barrier wall at the surface of droplets in emulsions. A common example is the reaction of a polyamine in one liquid phase with a polyisocyanate in another phase to form a polyurea wall surrounding a core containing an active. See, e.g., U.S. Pat. No. 5,525,595 to Seitz et al. However, the reactants that are suitable for such formulations are somewhat limited, and this can limit the types of active with which this technique can be successfully used. Furthermore, the production of such microcapsules having consistent properties requires careful control and expensive reactants.
Uniformly spherical particles, which demonstrate predictable and regular release rates, can also be provided by the formation of matrix microparticles. General information on the production of matrix microparticles can be found in Controlled release of pesticides from microparticles, Park, D. J., et al., Ch. 4, pp. 89-137, and in Dispersible microparticles, Smith, K. L., Ch. 5, pp. 137-149, both in Controlled-Release Delivery Systems for Pesticides, Scher, H. B., Ed., Marcel Dekker, Inc., New York (1999).
It is generally known that the release of a molecule, such as an agricultural active, from a matrix microparticle depends upon, among other things, the size and geometry of the particle and the compatibility between the active and the matrix material. Moreover, the compatibility between the active and the matrix material can also affect whether it is possible to successfully produce a useful matrix microparticle from a given active and a given matrix material. For example, if there is insufficient compatibility between the active and the matrix material, a majority of the active can be excluded from the matrix microparticle during the formation process. Such a product is characterized by a high concentration of the active present as crystals, or on the surface of the microparticles, and results in uncontrolled release of the active into the environment. A microparticle formulation having high levels of the active outside the particles, or on the surface of the particles, is usually found to have a high readily extractable active (REA) value.
Because of the often complex chemistries of modern agricultural actives, it has not been possible to predict a priori which combinations of active and matrix material can be expected to yield effective matrix microparticles having low REA values. For example, recently introduced chloronicotinyls have been shown to be useful as insecticides (See., e.g., U.S. Pat. Nos. 5,994,331, 6,077,860, 6,114,362), but their successful inclusion in controlled release forms that are capable of sustained release over periods longer than a few days has been difficult. See, e.g., Gonzalez-Pradas, E., Pestic. Sci., 55:546-552 (1999), and Fernandez-Perez, M., J. Agric. Food Chem., 46(9):3828-3834 (1998).
Because controlled release formulations that are designed for agricultural uses necessarily must be of a lower cost than, for example, medical applications, it is important to provide such formulations that can be produced economically and efficiently. Moreover, because such formulations are usually applied directly to plants or into the soil, it is important that the particles be biodegradable, so as not to persist in the environment.
Due to its wide availability and properties as a UV protectant, lignin has been used as a carrier or adjuvant for actives in agricultural compositions. For example, Dilling et al., in U.S. Pat. Nos. 4,751,247 and 4,797,157, describe the use of amine salts of ligonulfonates as a sequestrant in pesticide compositions. The use of alkali lignin as a pesticide dispersant was taught in U.S. Pat. Nos. 3,726,850 and 3,992,532. U.S. Pat. No. 3,813,236 described the covalent bonding of a pesticide to a lignin substrate, and U.S. Pat. No. 3,929,453, reissued as Re. No. 29,238, taught a slow release composite produced by co-precipitation of an alkali lignin or the removal of a common solvent from a lignin-pesticide mixture.
Other lignin-based sustained release formulations were described in U.S. Pat. Nos. 4,184,866, 4,244,728 and 4,244,729, each of which teaches the cross-linking of lignin with epichlorohydrin or formaldehyde.
In U.S. Pat. No. 4,381,194, the adsorption of a herbicide or fungicide onto particles of a water-insoluble alkali lignin and a surfactant, where the lignin had a mean particle size of from 0.5 to 5 microns in diameter. In U.S. Pat. Nos. 4,624,694 and 4,752,319, DelliColli described the use of a similar lignin slurry, except without the herbicide or fungicide, as a method of crop seed treatment to provide an increase in emergence of seedlings.
Lignosulfonates, in combination with a protein such as a high bloom gelatin, were reported in U.S. Pat. No. 5,552,149 to be useful for the formation of microcapsules that were resistant to UV degradation.
Other lignin derivatives, such as for example, lignin acetate, have been reported to be useful for applications such as acting as a binder in water-based printing ink compositions. (See, e.g., U.S. Pat. No. 4,612,051).
Accordingly, therefore, it would be useful to provide controlled release microparticles and formulations for agricultural actives that could be produced from readily available, biodegradable materials that would have a low environmental impact. It would also be useful if such microparticles would stabilize the active against UV degradation. Furthermore, it would be useful if such microparticles could be made to be sufficiently small so that they could be used effectively as components in a seed coating, but still capable of maintaining the release of the active over a period of time of several weeks, or months.